1. Field of the Invention
This invention relates to pleated filter elements, methods and apparatus for manufacturing pleated filter elements and methods for precoating and backwashing filter elements.
2. Description of the Related Art
Cylindrical filter elements having radially-extending, longitudinal pleats are among the most common types of filter elements and are used to filter innumerable fluids, i.e., liquids or gases. (Throughout this application, “filter” and “filtration” include both the removal of particulates, e.g., by sieving or trapping within a porous medium, and the removal of impurities, e.g., by ion exchange resins or sorbents). In a typical cylindrical pleated filter element, a plurality of pleats are arranged around a tubular core to define a cylinder. As viewed in a transverse cross-section, the individual pleats of such a filter element extend radially outward from the core toward the outer periphery of the filter element. It is good design practice to have enough pleats in a cylindrical filter element so that adjoining pleats contact one another along the circumference of the core. However, because of the radial geometry of the pleats, the spacing between adjacent pleats necessarily increases as the distance from the center of the core increases. Accordingly, in a typical cylindrical pleated filter element, there is a great deal of unused space between adjacent pleats.
Making a filter element larger to compensate for the unused space between adjacent pleats is frequently not possible. In the filter industry today, the dimensions of filter housings within which the filter elements are enclosed have become fairly standardized. Accordingly, a major challenge of filter designers is to increase the filtering capacity of a filter element, i.e., the usable surface area, without altering its external dimensions so that it can be employed with existing filter housings.
While cylindrical pleated filter elements are very common, they have typically not been used as precoat filters. A precoat filter is a type of filter in which a slurry is applied as a cake, called a precoat, to the exterior of a non-pleated, porous support structure called a septum. After the precoat is applied to the septum, a fluid to be filtered is then directed through the precoat and the septum where the precoat serves to filter the fluid. Pleated filter elements have not been used as supports for septa because the pleats tend to collapse as the precoat is applied or as the fluid flows through the precoat.